1. Field of the Invention
The present invention relates to a method for producing an optical fiber. More particularly, it relates to a method for producing an optical fiber having a surface of which is coated with an ultraviolet (UV)-curable resin.
2. Description of the Prior Arts
Conventionally, an optical fiber having a coating layer made of a UV-curable resin is produced by means of an apparatus for producing an optical fiber comprising a UV irradiating furnace as shown in FIG. 1. In the apparatus of FIG. 1, a preform 1 for an optical fiber is made molten by a heater 2 (for example, a carbon resistance heater) and drawn and spun to form an optical fiber 3. The optical fiber 3 is passed through a resin coating cup 4 in which a UV-curable resin is applied to the surface of the optical fiber 3 and then passed through a UV irradiating furnace 5 to cure the resin. The optical fiber coated with the cured resin is wound on a reel 11 through a capstan 10.
As shown in FIGS. 2A and 2B, the UV irradiating furnace 5 comprises an ellipsoidal reflection cylinder 6, a UV lamp 7 and a quartz tube in which a gas flowed to purge gases which prevent the curing of the UV-curable resin wherein the UV light 7 and the quartz tube are so arranged that each of their axes is aligned with each of the focus lines 6a and 6b of the ellipsoidal cylinder 6. When the optical fiber 3 applied with the UV-curable resin is passed through the quartz tube 8, the UV-curable resin is cured by focused irradiation of ultraviolet light emitted from the UV lamp 7.
According to the conventional method as described above, the UV-curable resin is evaporated by heat generated by the UV irradiating furnace during the optical fiber 3 is passed through the quartz tube 8 so that the evaporated resin is adhered and baked on the inner surface of the quartz tube and carbonized. Thereby, the amount of the UV light which passes through the wall of the quartz tube decreases as time passes so that the UV-curable resin on the surface of the optical fiber is not homogeneously cured along its length and the properties of the produced optical fiber are deteriorated.